The UK Independence party (Ukip) would only be prepared to make a pact with the Conservatives at the next general election if David Cameron's party made a promise "written in blood" to hold a referendum on Britain's membership of the EU, Nigel Farage has said.

Speaking ahead of his party's annual conference in Birmingham, the Ukip leader said that, while he hadn't offered a deal to any other party, he would be willing to consider one if either the Tories or Labour came to him with a "sensible" proposal.

He said many Conservatives were uneasy at Ukip's rise in the polls and feared they would be left "very exposed" at the next election. But any agreement would only be reached if there was "an absolute promise" that an in/out referendum would be held. Polls suggest that in the event of such a vote, Britain could leave the EU.

Farage told BBC Radio 4's Today programme: "I'm not offering a deal to anybody; all I'm saying is if the Conservative party or the Labour party came to me with a sensible deal that worked in the national interest we would sit down and talk about it."

Recent polls have given Ukip around 10% of the national vote – about the same as the Liberal Democrats. While its performance at first-past-the-post Westminister elections is not strong, observers say Ukip could potentially come second or even top in the 2014 European parliamentary elections, which use a proportional representation voting system.

Asked if Ukip would agree not to field candidates against Tory eurosceptics as part of any hypothetical deal, Farage refused to be drawn on the "specifics". But he said any agreement would be expected to get the party its first MP in the House of Commons.

He added: "They [the Tories] look at Ukip at 10% in the polls and they say: 'We've got to do a deal with Ukip.' So I'm responding to many voices in the Conservative party and what I'm saying is, we're a different party with a different manifesto but if we were offered a deal that made it easier to push open a door marked "independence" for the United Kingdom, of course we'd consider it.

"But I wouldn't even contemplate doing a deal, even if it gave the party advantage, unless we first had written in blood, I think, rather than a cast-iron guarantee, an absolute promise that we would have a proper referendum on our relationship with the EU."

He acknowledged that many in Ukip would find it hard to trust the Tories on the issue after Cameron "reneged" on an earlier vow to hold a referendum on the Lisbon treaty changes.

The prime minister is facing internal divisions over the EU as many of his own MPs push for a referendum on membership during the next parliament.